Ethylbenzene is one of the aromatic hydrocarbons that is obtained from naphtha pyrolysis or in reformate. Reformate is an aromatic product given by the catalysed conversion of straight-run hydrocarbons boiling in the 70 to 190° C. range, such as straight-run naphtha. Such hydrocarbons are themselves obtained by fractionation or distillation of crude petroleum oil, their composition varying depending on the source of the crude oil, but generally having a low aromatics content. On conversion to reformate, the aromatics content is considerably increased and the resulting hydrocarbon mixture becomes highly desirable as a source of valuable chemicals intermediates and as a component for gasoline. The principle components are a group of aromatics often referred to as BTX: benzene, toluene, and the xylenes, including ethylbenzene. Other components may be present such as their hydrogenated homologues, e.g. cyclohexane.
Of the BTX group the most valuable components are benzene and the xylenes, and therefore BTX is often subjected to processing to increase the proportion of those two aromatics: hydrodealkylation of toluene to benzene and toluene disproportionation to benzene and xylenes. Within the xylenes, para-xylene is the most useful commodity and xylene isomerisation or transalkylation processes have been developed to increase the proportion of para-xylene.
A further process that the gasoline producer can utilize is the hydrodealkylation of ethylbenzene to benzene.
Generally, the gasoline producer will isolate BTX from the reformate stream, and then subject the BTX stream to xylene isomerisation with the aim of maximising the para-xylene component. Xylene isomerisation is a catalytic process; some catalysts used in this process have the ability not just to isomerise xylenes but also simultaneously to dealkylate the ethylbenzene component. Normally the para-xylene is then separated out to leave benzene, toluene (unless toluene conversion processes have already been applied) and the remaining mixed xylenes, including ethylbenzene. This BTX stream can either be converted by transalkylation to increase the yield of xylenes by contacting with a heavier hydrocarbon steam or can be converted by dealkylation to eliminate selectively ethylbenzene and to increase the yield of benzene, while allowing the xylenes to reach equilibrium concentrations. The latter process is the subject of the present invention.
In ethylbenzene dealkylation at this latter stage of BTX treatment, it is a primary concern to ensure not just a high degree of conversion to benzene but also to avoid xylene loss. Xylenes may typically be lost due to transalkylation, e.g. between benzene and xylene to give toluene, or by addition of hydrogen to form, for example, alkenes or alkanes.
It is therefore the aim of the present invention to provide a catalyst that will convert ethylbenzene to benzene with a high selectivity without xylene loss. Simultaneous xylene isomerisation to equilibrium concentrations is also desirable.
The catalysts used for the production of reformate are often platinum-on-alumina catalysts. For the conversion of BTX streams to increase the proportion of closely configured molecules, a wide range of proposals utilizing zeolitic catalysts have been made, which include those of EP-A-0 018 498, EP-A-0 425 712, and WO 00/38834.
European Patent Specification No. 0 018 498 A1 is concerned with catalysts suitable for xylene isomerisation and the simultaneous dealkylation of ethylbenzene and reviews a number of earlier proposals for the use of platinum ZSM-series zeolitic catalysts. Generally such catalysts are shown to have a superior activity in isomerising xylenes and to dealkylate ethylbenzene, but are required to be used at high temperatures as there is a tendency for platinum to hydrogenate the benzene ring and to cause other undesirable side-reactions such as disproportionation and transalkylation at the low temperatures that are preferred for xylene isomerisation. The proposal of EP-A-0 018 498 is to use a second metal, which is preferably tin, barium, titanium, indium and cadmium, in combination with platinum and a high-silica zeolite bound with a refractory inorganic oxide, which in all of the examples is alumina.
EP-A-0 425 712 describes an improved catalyst for simultaneous xylene isomerisation and ethylbenzene dealkylation, which is formed by combining a group VIII metal, preferably platinum, with a lead component, and a halogen component, on a carrier of a pentasil zeolite and an inorganic oxide binder, preferably alumina, such that a specific ratio of lead to Group VIII metal is achieved and such that the bulk of the Group VIII and lead components are combined with the binder material.
WO 00/38834 describes a mixed zeolitic catalyst for the disproportionation and transalkylation of aromatic hydrocarbons. That catalyst consists of a carrier of 10 to 80 wt % mordenite and/or zeolite beta, 0 to 70 wt % ZSM-5, and 5 to 90 wt % inorganic binder, plus a metal component of platinum with either tin or lead. While the binder is said to be most preferably alumina or silica, only alumina-bound catalysts are exemplified.
There are fewer proposals for catalysts directed solely for the hydrodealkylation of aromatics.
Toppi et al in Journal of Catalysis 210, 431-444 (2002) studies the use of silica-supported platinum and platinum-tin catalysts in comparison with acidic catalysts of just alumina and chlorinated alumina, on the hydrodealkylation of n-propylbenzene, and finds that the formation rate of benzene was the highest for the acidic catalysts.
U.S. Pat. No. 3,992,468 proposes two catalysts for the hydrodealkylation of alkylaromatic hydrocarbons: catalyst A essentially containing a) a carrier, b) at least one metal selected from the group consisting of the metals from group VIII and c) at least one metal selected from the group consisting of zinc, cadmium, gallium, indium, thallium, copper, silver, gold, yttrium, titanium, niobium, tantalum, and manganese; and catalyst B essentially containing a) a carrier, b) at least one metal selected from a first group consisting of chromium, molybdenum, tungsten, rhenium, and manganese, and c) at least one additional metal different to that of the first group and being selected from the metals of the first group plus copper, silver, gold, zinc, cadmium, gallium, indium, thallium, germanium, tin and lead, each metal being in an amount of from 0.05 to 20 wt %. The carrier is selected from among known carriers, for example, alumina, magnesia, magnesia-silica, acidic alumina, chlorinated and/or fluorinated alumina, alumina-silica, zirconia, zirconia-silica, and molecular sieves or zeolites, and is preferably alumina.